1. Field of the Invention
This invention relates generally to communication systems, and, more particularly, to fans used in communication systems.
2. Description of the Related Art
Communication equipment such as base stations, network controllers, switching centers, routers, and the like generate heat when electrical current is dissipated within the equipment. Consequently, most communication equipment includes one or more fans that are used to cool the equipment to maintain a selected operating temperature or range of temperatures. The fans are typically powered by one or more direct current (DC) motors to draw power from a power supply, such as a 48 V power supply. The fan motors usually have two poles and so the fan motor draws two pulses of DC current from the power supply for each revolution of the fan. Depending on the angular frequency of the fan, the current pulse rate for fan motors that are used in communication equipment usually varies between about 100 and 300 Hz, although in some cases the current pulse rate can fall outside of this range.
The current pulses drawn by the fan motor can generate electrical noise at the current pulse frequency and at the harmonics of the pulse frequency. The noise produced by these current pulses can couple onto the 48 V power supply and then be transferred directly on to any communication lines that are connected to the communication equipment including the fan motor. For example, in applications where plain old telephone service (POTS) is employed, the current pulse noise can couple onto the 48 V power supply and then directly onto the POTS lines that carry communication signals. The electrical noise produced by the current pulses can affect the audible noise perceived by a person that is using telephone equipment that is coupled to the communication lines, particularly if the electrical noise has a frequency that falls between about 300 Hz and 4 kHz. Thus, harmonics of the current pulse frequency may produce audible noise in a POTS communication system, as well as in other communication systems. Consequently, service providers have set very stringent requirements on the amount of electrical noise that can be produced by a fan motor used in communication equipment. For example, in a POTS system, the psophometric noise must be less than 0.4 mV.
Meeting these requirements on the amount of electrical noise produced by a fan motor that can couple to the 48 V power supply has been problematic for at least two reasons. First, since the frequency of the current pulses and/or harmonics of the current pulse frequencies can follow the audio range used by indications systems such as POTS communication systems, the tolerable level of noise is very low, which results in even more stringent limitations on the amount of acceptable noise that can be produced by a fan motor. Second, the relatively low-frequency electrical noise produced by the fan motors requires relatively large passive filters to achieve sufficient attenuation to meet the noise requirements. Often times, fan manufacturers do not appreciate the effort needed to meet the noise requirements until a prototype of the fan has been designed, built, and tested. Consequently, the design of the fan may not include sufficient room to accommodate the required large filter components, which may result in significant additional costs for re-design, re-testing, and/or re-production of the fans.